Effects of alertness on three-dimensional eye movements

Purpose: To investigate effects of alertness on three-dimensional (3D) eye movements. Methods: During drowsy and alert periods, 3D eye movements were recorded wi th dual search coils in three normal rhesus monkeys, and in two of these mo nkeys after placement of bilateral kainic acid lesions of the rostral inter stitial nucleus of the medial longitudinal fasciculus (riMLF). Results: When the three monkeys were in the normal state, the average plane thickness (standard deviation of all rotation vectors from the regressed p lane) during drowsy periods (range, 1.2-1.9 degrees) increased significantl y compared with the plane thickness during alert periods (range, 0.7-0.9 de grees) in the light (P < .05). The plane thickness increased significantly in the monkey with asymmetric bilateral riMLF-lesions (P < .05); however, t he increase was not significant in the monkey with more symmetric bilateral riMLF-lesions. After the bilateral riMLF-lesions, the plane thickness also increased with drowsiness in both monkeys (P < .02), as it had in the norm al state. Conclusion: The implementation of Listing's law is independent of the prese nce of vertical-torsional burst neurons in the riMLF. The increase in thick ness of Listing's plane during drowsy periods is not due to an imprecise si gnal from saccadic burst neurons to the 3D velocity-to-position integrator. It is possible that this integrator itself controls Listing's law, dependi ng on the state of alertness. (C) 2000 Japanese Ophthalmological Society.